The invention relates to a data exchange network and more particularly to a communication system of data exchange network suitable applicable to one-way transmission of large capacity document data and image data.
There are two types of data exchange network, a packet switched network and a circuit switched network. In the packet switched network, since charges are calculated according to the number of packets used, this network is advantageous when the transmission data occurs only occasionally. On the other hand, in the circuit switched network, charging is made according to the time that the circuit is appropriated, and therefore this network is not advantageous for sporadic data transmission. However, the circuit switched network is capable of a transparent transmission and therefore does not require such complicated controls as required by the packet switched network in sending a large volume of data, permitting a rapid and highly efficient data transmission.
To make most of the features of the above two types of communication network, a communication system was devised as disclosed by the Japanese Patent Laid-Open No. 195040/1986. This is shown in FIG. 2. In this example, when a large volume of data is to be transmitted from the packet terminal 21b, a call setup packet sets a flag requesting the use of the circuit switched network. Equipment 27 attached to the packet concentrator 25 identifies this flag to secure a path through the circuit switched network, that is, through the circuit concentrators 23, 23' and circuit switches 24, 24' to an add-on equipment 27', a packet concentrator 25' and a terminal 21b on the receiving side. When the circuit switched network appropriation request flag is not set, the packet transmission is performed through a packet switches 26, 26'.
The above conventional communication technique is effective for conventional data transmission, for example, between TSS terminals and computer and for copying of the contents of magnetic tapes. This conventional system, however, does not include any provision for new types of data transmission that will emerge as high speed, wide band communication network advances.
For example, search of data base is generally performed by a computer that manages the data base. However, as the storage devices such as semiconductor memories and magnetic disks become less expensive, it is more efficient to transmit a large capacity of data to the user side where the search is done by the work station on the user side because this enables diversified processing of data.
As so-called electronic publishing which uses the high-speed wide-band network has come into practical use, there are growing demands for a technique that can transfer several pages of document data or image data without giving an operator an impression of system action delay with respect to the user command.
As mentioned above, in the high-speed wide-band network, it is necessary not only to send a large volume of information in a short period of time but also to transmit it intermittently in response to the user command. While it is possible to cope with the above requirements by using the circuit switched network, it is not economical because the large-capacity high-speed communication line is used only sporadically. On the other hand, the packet switched network has a certain limit to the increased capacity and speed because of the complexity of its control.